Reduce allocations in Microsoft.Extensions.Configuration

[prozolic]

Give the provider lists an initial capacity so List<T> does not reallocate its backing array while it grows in the Microsoft.Extensions.Configuration APIs.

Changed:

• ConfigurationBuilder.Build
• ConfigurationManager.ReloadSources
• ReferenceCountedProviderManager.AddProvider

Benchmark

Method	Job	Toolchain	SourceCount	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Gen1	Allocated	Alloc Ratio
Builder_Build	Job-TDYACH	\main\corerun.exe	4	918.1 ns	17.28 ns	19.90 ns	1.00	0.00	0.2956	0.0019	2480 B	1.00
Builder_Build	Job-DXRHTD	\pr\corerun.exe	4	838.9 ns	16.61 ns	35.76 ns	0.91	0.04	0.2956	0.0019	2480 B	1.00
Manager_AddSources	Job-TDYACH	\main\corerun.exe	4	2,032.7 ns	40.63 ns	93.36 ns	1.00	0.00	0.5379	0.0057	4512 B	1.00
Manager_AddSources	Job-DXRHTD	\pr\corerun.exe	4	1,812.1 ns	31.30 ns	41.79 ns	0.89	0.05	0.5093	0.0057	4264 B	0.95
Manager_ReloadSources	Job-TDYACH	\main\corerun.exe	4	1,329.2 ns	26.31 ns	70.69 ns	1.00	0.00	0.3548	0.0038	2968 B	1.00
Manager_ReloadSources	Job-DXRHTD	\pr\corerun.exe	4	1,261.4 ns	24.85 ns	37.19 ns	0.95	0.06	0.3452	0.0038	2888 B	0.97
Builder_Build	Job-TDYACH	\main\corerun.exe	16	3,169.3 ns	62.90 ns	92.20 ns	1.00	0.00	1.1024	0.0343	9248 B	1.00
Builder_Build	Job-DXRHTD	\pr\corerun.exe	16	2,959.3 ns	58.84 ns	130.39 ns	0.93	0.05	1.0872	0.0343	9104 B	0.98
Manager_AddSources	Job-TDYACH	\main\corerun.exe	16	5,783.2 ns	114.55 ns	301.78 ns	1.00	0.00	1.9989	0.0687	16752 B	1.00
Manager_AddSources	Job-DXRHTD	\pr\corerun.exe	16	6,129.9 ns	122.14 ns	268.11 ns	1.06	0.07	1.7090	0.0610	14296 B	0.85
Manager_ReloadSources	Job-TDYACH	\main\corerun.exe	16	3,882.5 ns	97.60 ns	286.25 ns	1.00	0.00	1.1749	0.0572	9832 B	1.00
Manager_ReloadSources	Job-DXRHTD	\pr\corerun.exe	16	3,892.0 ns	81.11 ns	239.16 ns	1.01	0.09	1.1253	0.0534	9416 B	0.96
Builder_Build	Job-TDYACH	\main\corerun.exe	64	13,280.9 ns	260.36 ns	482.59 ns	1.00	0.00	4.3182	0.5035	36176 B	1.00
Builder_Build	Job-DXRHTD	\pr\corerun.exe	64	12,785.1 ns	253.18 ns	371.11 ns	0.96	0.04	4.2419	0.5035	35600 B	0.98
Manager_AddSources	Job-TDYACH	\main\corerun.exe	64	34,694.8 ns	628.64 ns	672.64 ns	1.00	0.00	11.9324	1.5259	99984 B	1.00
Manager_AddSources	Job-DXRHTD	\pr\corerun.exe	64	25,616.3 ns	512.28 ns	1,246.95 ns	0.74	0.04	7.8430	1.0071	65656 B	0.66
Manager_ReloadSources	Job-TDYACH	\main\corerun.exe	64	15,124.2 ns	281.92 ns	301.65 ns	1.00	0.00	4.4250	0.7324	37144 B	1.00
Manager_ReloadSources	Job-DXRHTD	\pr\corerun.exe	64	14,977.4 ns	299.17 ns	766.89 ns	0.99	0.05	4.2419	0.7019	35528 B	0.96

Benchmark source

public sealed class ByteSizeConfig : ManualConfig
{
    public ByteSizeConfig()
    {
        SummaryStyle = SummaryStyle.Default.WithSizeUnit(SizeUnit.B);
        AddDiagnoser(MemoryDiagnoser.Default);
    }
}

[Config(typeof(ByteSizeConfig))]
public class ConfigBench
{
    [Params(4, 16, 64)]
    public int SourceCount { get; set; }

    private MemoryConfigurationSource[] _sources;
    private ConfigurationBuilder _builder;
    private ConfigurationManager _managerForReload;

    [GlobalSetup]
    public void Setup()
    {
        Assembly asm = typeof(ConfigurationBuilder).Assembly;
        Console.WriteLine($"[assembly] {asm.Location}");
        Console.WriteLine($"[mvid]     {asm.ManifestModule.ModuleVersionId}");

        _sources = new MemoryConfigurationSource[SourceCount];
        for (int i = 0; i < SourceCount; i++)
        {
            _sources[i] = new MemoryConfigurationSource();
        }

        _builder = new ConfigurationBuilder();
        foreach (MemoryConfigurationSource source in _sources)
        {
            _builder.Add(source);
        }

        _managerForReload = new ConfigurationManager();
        foreach (MemoryConfigurationSource source in _sources)
        {
            _managerForReload.Sources.Add(source);
        }
    }

    [GlobalCleanup]
    public void Cleanup() => _managerForReload?.Dispose();

    [Benchmark]
    public IConfigurationRoot Builder_Build() => _builder.Build();

    [Benchmark]
    public ConfigurationManager Manager_AddSources()
    {
        var manager = new ConfigurationManager();
        for (int i = 0; i < _sources.Length; i++)
        {
            manager.Sources.Add(_sources[i]);
        }
        return manager;
    }

    [Benchmark]
    public ConfigurationManager Manager_ReloadSources()
    {
        _managerForReload.Sources[0] = _sources[0];
        return _managerForReload;
    }
}

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[Copilot]

🟢 Approval recommended

The changes are small, localized, and preserve existing behavior while applying a standard allocation-avoidance pattern (pre-sizing lists when the final count is known).

Pull request overview

This PR reduces transient allocations in Microsoft.Extensions.Configuration by pre-sizing provider lists based on the known source count, avoiding List<T> growth reallocations in common paths.

Changes:

• Pre-allocates List<IConfigurationProvider> capacity in ConfigurationBuilder.Build() using _sources.Count.
• Pre-allocates List<IConfigurationProvider> capacity in ConfigurationManager.ReloadSources() using _sources.Count.
• Updates ReferenceCountedProviderManager.AddProvider() to copy providers into a new list with Count + 1 capacity before appending.

File summaries

File	Description
src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationBuilder.cs	Pre-sizes providers list to _sources.Count before building providers.
src/libraries/Microsoft.Extensions.Configuration/src/ConfigurationManager.cs	Pre-sizes the rebuilt providers list during ReloadSources() to _sources.Count.
src/libraries/Microsoft.Extensions.Configuration/src/ReferenceCountedProvidersManager.cs	Avoids list growth reallocation by allocating providers.Count + 1 before copying and adding a provider.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

[Copilot]

🟢 Approval recommended

The changes are low-risk, preserve existing behavior, and align with the intended allocation reduction by pre-sizing provider lists where the final size is already known.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Lite

[rosebyte]

LGTM

Thank you for the contribution!

[Copilot]

🔵 Needs a closer look

One or more issues must be addressed before approval.

Review details

Suppressed comments (1)

src/libraries/Microsoft.Extensions.Configuration/src/ReferenceCountedProvidersManager.cs:71

• The supplied benchmark shows a throughput regression for this path at the intermediate size: Manager_AddSources goes from 5,783.2 ns to 6,129.9 ns for 16 sources (ratio 1.06), even though allocations improve. Please benchmark an explicit List<IConfigurationProvider> with capacity existing.Count + 1, followed by AddRange and Add, to retain the allocation win without the collection-expression lowering overhead before merging.

                _refCountedProviders.Providers = [.. _refCountedProviders.Providers, provider];

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Lite